The physics of banging your head

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Call me Jeremy, because I spoke on reddit the other day. And I got the expected backlash. This was augmented by the fact that I spoke about… wait for it… bike helmets. Nothing will get folks worked up as quickly as bike helmets and cyclists rolling through stop signs. But I just can’t seem to keep myself out of helmet discussions.

However, I just happen to be the type of guy that can find value in someone telling me I’m a complete idiot. And in this case the “value” was this blog post.

The reddit post was positioned as a PSA (that’s Public Service Announcement for those of you not into TLAs.) The original poster (That’s OP, which is also a TLA, only it is spelled out as two letter acronym and never as TLA) related a story about how he had crashed and got a laceration on his face. His advice as a result: “Wear your helmet.” Then in the comments, someone had to offer this pearl of wisdom:

Unfortunately the more we ride bikes, we are all going to crash at some point.

Some dude(tte) on reddit

My aversion to the culture of fear that tries to paint cycling as inherently dangerous kicked in. Therefore I, of course, had to offer a counter-perspective:

Funny how no one ever says “The more we walk, the more we are going to trip over something and hit our heads. Be sure to wear a helmet while walking!”

The reddit user formerly known as PleaseTrollMe

I walked away from the keyboard smug as a bug in a rug. Someone had been wrong on the internet and I’d set them right.

…until someone responded to my comment.

But, much to my surprise, the response didn’t use a single profanity. It didn’t feel ranty. It actually had the air of being well thought out. It made a point. I felt certain that the comment was actually made by a rational, sane, level headed reddit user.

If we walked 15-25 mph they probably would.

Reasonably sane reddit user

Huh…. Ok…

The comment, while sounding very commonsensey, actually points to one of the great misconceptions about bicycle helmets. Well, actually two misconceptions. First, it assumes that helmets are designed to protect against 20-25 MPH impacts. Second, it assumes that the speed you are traveling changes the force with which your head hits the road if you do crash. Let’s look at both of these.

Bicycle helmets may not be protecting you from what you think

In the US, bicycle helmets are currently required to meet standards laid out in the late 90’s by the Consumer Product Safety Commission. Commonly referred to simply as “The CSCP,” the standard legally applies to all helmets sold in the United States after March 10, 1999. And the standard is extremely straight forward:

The CPSC standard uses a lab test drop of 2.0 meters on a flat anvil and 1.2 meters on a hemispheric and a curbstone anvil.


Wait… a drop of 2.0 meters?!?!?! That’s right. Helmets are tested by placing them on a head-like thingy full of accelerometers and other measuring devices, and then dropping them from a height of 2 meters. Not rushed towards. Not impacted with. Simply dropped. You can get those images of crash testing cars out of your head. A helmet is simply dropped. The 2 meters is about 6 1/2 feet for those that find metric numbers weird. Or about the hight of a sort of tall person. So the test is equivalent to a standing person simply falling over backwards.

For many of you this will seem absolutely ridiculous. After all, except for the inevitable first fall while learning how to use your clipless pedals, it is rare that someone simply falls over on their bike. Instead, we go flying over the handlebars, or keel over sideways while pedaling.

But that takes us to our second misconception.

Your head doesn’t hit the ground in the way you may think

Physics can be hard and counter-intuitive. This was demonstrated plainly when astronauts on the moon recreated Galileo’s famous drop experiment. On the moon, in the absence of air to produce drag, a feather and a hammer both fell at the exact same speed. Which for many people feels like it contradicts what their “common sense” tells them.

But back to our 25 mph cyclist. Well, equally counter-intuitive for some is the fact that the speed your are riding at has absolutely no relevance to how hard your head hits the road. I’m sure there are some of you reading this that are now jumping on the “This guy is actually JustAnotherIdiot” train. Of course you hit your head harder when riding at 25 MPH! Actually no, you don’t.

First off – obviously here I’m talking only about an individual crashing in a way that causes their head hits the road. Ride your bike 25 MPH straight into a brick wall then yes, your head hits the wall at 25 MPH (more on this below.) However, when you crash your head actually only hits the ground due to the force of gravity. Your forward momentum is completely different from your downward momentum. Since the impact force is due to the downward pull of gravity only, you are, in fact, sustaining exactly the same impact you would if you were simply standing and fell over. There is a reason that bike helmets are tested by dropping them from 2 meters. That accurately simulates the actual impact forces in the most likely impact scenario on your bike – smacking your noggin on the tarmac.

And that is precisely why, in all seriousness, I repeat:

Funny how no one ever says “The more we walk, the more we are going to trip over something and hit our heads. Be sure to wear a helmet while walking!”

Usually about this point in the conversation someone will state something along the lines of “Well I’m sure the helmet manufactures make helmets better than these bare minimums.” In response I usually counter with challenge: Find me a single bicycle helmet ad that makes any claims about the safety or effectiveness of their helmets. I know I’ve never found a single one. Car manufactures tout the safety of their models constantly – even putting graphic, slow-motion video of crash tests in their commercials. Bike helmet ads seem to center around light weight, aerodynamics or style only.

The absence of the mention of safety in bike helmets ads is not accidental. The truth is the kind of protection that so many people mistakenly believe their helmets provide isn’t achievable in a light weight, airflow friendly package. To protect agains a concussion, for example, you’d need the equivalent of a car’s crumple zone around your head. A concussion is caused by your skull stopping abruptly, but your brain inside your skull continuing to move and smacking agains the inside of your skull. To prevent this, you need something to absorb the impact and slow the rate at which the skull stops. In plain terms, a whole lot more styrofoam that is soft enough to absorb the impact.

By not mentioning safety in their advertising the bike helmet manufactures get to take the benefit of all the false assumptions people make about bike helmet protection, without making any specific claims at all. There is no reason for helmet manufactures to say anything about safety, because the vast majority of the population already mistakenly believes that bike helmets provide a whole lot more protection than the actually do.

So next time you feel the urge to perpetuate the idea that it is ridiculous to ever ride a bike without a helmet, consider that you are assuming precisely the same risk every time you stand up in your living room.

  • Great post Ross. I agree completely. My choice to wear a helmet, regardless of how effective they are or aren’t. My personal choice. Same with motorcycle helmets and seatbelts. I choose to wear these things and take these precautions, but it is my choice completely. Your point that there are many actions that we could take as a society that would actually keep cyclists more safe is right on – and that’s where we should focus our collective efforts. If I choose to do stupid things, leave me alone, but let’s focus our collective efforts on safety for others.

    • Pete S

      Funny I just happen to come across this article. I am an avid bike rider and commuter. I ride to the bus, then get off the bus and ride the rest of the way to work. There have been a few times, where I left my garage and get a few houses down the street and realize I forgot my helmet. Up until 4 days I would continue this practice. That day, at lunch I left my office on foot to go grab a salad across the street. About 30 seconds before I got there, another person with the same idea was coming back across the street, on foot, in a cross walk, and got hit by a car, knocked up into the air and apparently landed on his head. He ended up a good 40 ft from the cross walk. No broken bones except for his orbital socket, and a big cut on the back side of his head. It was pretty gruesome, I had to hold him down until the paramedics got there. He was obviously in shock, trying to get up. Talking about a head injuries, imagining what a head injury is like is not the same as seeing one first hand. It was pretty bad. As I was holding him while waiting for the paramedics, I found my self looking at him trying to picture how it would have been less severe had he been wearing a helmet. I realized he was walking and obviously no one would consider wearing one for a walk, but i found my self trying to gauge how much less the injuries would have been had he been wearing a helmet. After seeing that, I will always wear a helmet on every single bike ride. If I forget to wear one, I will turn around and go back and get it. I am telling you, seeing it first hand is convincing. Anyways thats my two cents.